Answered by Shaykh Yusuf Weltch
Question: What is the textual evidence that prayer is not permissible when the sun is at its zenith?
Answer: In the Name of Allah, the Most Merciful and Compassionate
The Messenger of Allah said, “The prayer is witnessed and attended (i.e. by Angels) until the mid-day and it is certainly a time when the doors of Hell are opened and its fire kindled. So leave the prayer until the shadow appears. [Nasaai]
Uqbah bin Amir al-Juhani narrates, “There are three times which the Messenger of Allah (may Allah bless him and give him peace) would prohibit us from praying or burying our deceased therein: When the sun clearly begins to rise until it has fully risen, when the sun reaches its peak till it begins to decline, and when the sun’s light begins to weaken till it fully sets.” [Muslim]
There are many other narrations in this regard as well.
May Allah bless you
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani